The efficiency of a heating boiler is the ratio of useful heat consumed to produce steam (or hot water), to the available heat of the heating boiler. Not all the useful heat generated by the boiler unit is sent to consumers; part of the heat is spent on its own needs. Taking this into account, the efficiency of a heating boiler is distinguished by the heat generated (gross efficiency) and by the heat released (net efficiency).

The difference between the generated and released heat is used to determine the consumption for auxiliary needs. Not only heat is consumed for its own needs, but also electrical energy (for example, to drive a smoke exhauster, fan, feed pumps, fuel supply mechanisms), i.e. consumption for own needs includes the consumption of all types of energy spent on the production of steam or hot water.

As a result, the gross efficiency of a heating boiler characterizes the degree of its technical perfection, and the net efficiency characterizes its commercial profitability. For a boiler unit gross efficiency, %:
according to the direct balance equation:

η br = 100 Q floor / Q r r

where Q floor is the amount of useful heat, MJ/kg; Q р р — available heat, MJ/kg;

according to the reverse balance equation:

η br = 100 - (q u.g + q h.n + q n.o)

where q u.g, q h.n, q n.o - relative heat losses with exhaust gases, from chemical incomplete combustion of fuel, from external cooling.

Then the net efficiency of the heating boiler according to the reverse balance equation:

η net = η br - q s.n

where q s.n is energy consumption for own needs, %.

The determination of efficiency using the direct balance equation is carried out mainly when reporting for a separate period (decade, month), and using the reverse balance equation - when testing a heating boiler. Calculating the efficiency of a heating boiler using reverse balance is much more accurate, since the errors in measuring heat losses are smaller than in determining fuel consumption.

Dependence of boiler efficiency η k on its load (D/D nom) 100

q u.g, q h.n, q n.o - heat losses with exhaust gases, from chemical and mechanical incomplete combustion, from external cooling and total losses.

Thus, to improve the efficiency of a heating boiler, it is not enough to strive to reduce heat losses; It is also necessary to completely reduce the consumption of thermal and electrical energy for own needs, which amount on average to 3...5% of the heat available in the boiler unit.

The change in efficiency of a heating boiler depends on its load. To construct this dependence (Fig.), you need to subtract from 100% sequentially all losses of the boiler unit, which depend on the load, i.e. q u.g, q x.n, q n.o. As can be seen from the figure, the efficiency of a heating boiler at a certain load has a maximum value. Operating the boiler at this load is most economical.

There are 2 methods for determining efficiency:

By direct balance;

By reverse balance.

Determining the efficiency of a boiler as the ratio of useful heat expended to the available heat of the fuel is its determination by direct balance:

The boiler efficiency can also be determined by the reverse balance - through heat losses. For the steady thermal state we obtain

. (4.2)

The boiler efficiency, determined by formulas (1) or (2), does not take into account electrical energy and heat for its own needs. This boiler efficiency is called Gross efficiency and denote or .

If the energy consumption per unit of time for the specified auxiliary equipment is , MJ, and the specific fuel consumption for electricity generation is, kg/MJ, then the efficiency of the boiler plant taking into account energy consumption auxiliary equipment (Net efficiency), %,

. (4.3)

Sometimes called the energy efficiency of a boiler plant.

For boiler installations industrial enterprises Energy costs for own needs account for about 4% of generated energy.

Fuel consumption is determined:

Determination of fuel consumption is associated with a large error, so the efficiency by direct balance is characterized by low accuracy. This method is used to test an existing boiler.

The reverse balance method is characterized by greater accuracy and is used in the operation and design of the boiler. In this case, Q 3 and Q 4 are determined according to recommendations and from reference books. Q 5 is determined from the graph. Q 6 is calculated (rarely taken into account), and essentially the determination by reverse balance comes down to the determination of Q 2, which depends on the temperature of the flue gases.

The gross efficiency depends on the type and power of the boiler, i.e. productivity, type of fuel burned, firebox design. The efficiency is also affected by the boiler operating mode and the cleanliness of the heating surfaces.

In the presence of mechanical underburning, part of the fuel does not burn (q 4), and therefore does not consume air, does not form combustion products and does not release heat, therefore, when calculating the boiler, the calculated fuel consumption is used

. (4.5)

Gross efficiency only takes into account heat losses.

Figure 4.1 - Change in boiler efficiency with load change

5 DETERMINATION OF HEAT LOSS IN A BOILER UNIT.

WAYS TO REDUCE HEAT LOSS

5.1 Heat loss with flue gases

Heat loss with exhaust gases Q y.g occurs due to the fact that the physical heat (enthalpy) of the gases leaving the boiler exceeds the physical heat of air and fuel entering the boiler.

If we neglect the small value of the enthalpy of the fuel, as well as the heat of the ash contained in the flue gases, the heat loss with the flue gases, MJ/kg, is calculated by the formula:

Q 2 = J ch.g - J c; (5.8)

where is the enthalpy of cold air at a=1;

100-q 4 – proportion of burned fuel;

a с.г – coefficient of excess air in the flue gases.

If the temperature environment is equal to zero (t x.v = 0), then the heat loss with the exhaust gases is equal to the enthalpy of the exhaust gases Q у.г =J у.г.

Heat loss with flue gases usually occupies the main place among the heat losses of the boiler, amounting to 5-12% of the available heat of the fuel, and is determined by the volume and composition of combustion products, which significantly depend on the ballast components of the fuel and on the temperature of the flue gases:

The ratio characterizing the quality of the fuel shows the relative yield of gaseous combustion products (at a = 1) per unit heat of combustion of the fuel and depends on the content of ballast components in it:

– for solid and liquid fuels: moisture W Р and ash А Р;

– for gaseous fuel: N 2, CO 2, O 2.

With an increase in the content of ballast components in the fuel and, consequently, the loss of heat with exhaust gases increases accordingly.

One of the possible ways to reduce heat loss with flue gases is to reduce the coefficient of excess air in the flue gases a c.g., which depends on the air flow rate in the furnace a T and the ballast air sucked into the boiler flues, which are usually under vacuum

a y.g = a T + Da. (5.10)

In boilers operating under pressure, there are no air suctions.

With a decrease in a T, the heat loss Q y.g decreases, however, due to a decrease in the amount of air supplied to the combustion chamber, another loss may occur - from the chemical incompleteness of combustion Q 3.

Optimal value a T is selected taking into account the achievement of the minimum value q y.g + q 3.

The decrease in a T depends on the type of fuel burned and the type of combustion device. With more favorable conditions contacting fuel and air, the excess air a T necessary to achieve the most complete combustion can be reduced.

Ballast air in combustion products, in addition to increasing heat loss Q.g., also leads to additional costs electricity for the smoke exhauster.

The most important factor influencing Q a.g. is the temperature of the exhaust gases t a.g. Its reduction is achieved by installing heat-using elements (economizer, air heater) in the tail part of the boiler. The lower the temperature of the exhaust gases and, accordingly, the lower the temperature difference Dt between the gases and the heated working fluid, the big square surface H is required for the same gas cooling. An increase in t c.g leads to an increase in losses from Q c.g and to additional fuel costs DB. In this regard, the optimal t c.g is determined on the basis of technical and economic calculations when comparing annual costs for heat-using elements and fuel for different meanings t h.g.

In Fig. 4 we can highlight the temperature range (from to ), in which the calculated costs differ slightly. This gives grounds for choosing the most appropriate temperature, at which the initial capital costs will be lower.

There are limiting factors when choosing the optimal one:

A) low temperature corrosion tail surfaces;

b) when 0 C it is possible for water vapor to condense and combine with sulfur oxides;

c) the choice depends on the temperature of the feed water, the air temperature at the inlet to the air heater and other factors;

d) contamination of the heating surface. This leads to a decrease in the heat transfer coefficient and an increase.

When determining heat loss with flue gases, the reduction in gas volume is taken into account

. (5.11)

5.2 Heat loss from chemical incomplete combustion

Heat loss from chemical incomplete combustion Q 3 occurs when fuel is incompletely burned within the combustion chamber of the boiler and flammable gaseous components CO, H 2 , CH 4 , C m H n appear in the combustion products... The combustion of these combustible gases outside the furnace is practically impossible because -due to their relatively low temperature.

Chemical incomplete combustion of fuel can result from:

– general lack of air;

– poor mixture formation;

– small size of the combustion chamber;

– low temperature in the combustion chamber;

– high temperature.

With enough for complete combustion fuel air quality and good mixture formation q 3 depends on the volumetric density of heat release in the furnace

The optimal ratio at which the loss of q 3 has a minimum value depends on the type of fuel, the method of its combustion and the design of the furnace. For modern combustion devices, the heat loss from q 3 is 0÷2% at q v =0.1÷0.3 MW/m 3.

To reduce heat loss from q 3 in the combustion chamber, they strive to increase the temperature level, using, in particular, heating the air, as well as improving the mixing of combustion components in every possible way.

For a modern boiler room liquid fuel Efficiency will often reach 80%, provided that the boiler room is clean and free of soot. However, the real efficiency on average (for those boiler houses that were measured) is approximately 65%. More often than not, the boiler room is not clean enough to accept heat from the flame and transfer maximum amount warm water.

The situation is much more complicated when boiler house manufacturers begin to talk about efficiency reaching 95%. It is not clear what conditions were used to determine the efficiency, and what efficiency is meant.

In the technical/economic field, at least 6 definitions are used for boiler room efficiency. Since many people do not know the conditions for determining the efficiency of a boiler room, suppliers, without fear of being accused of lying, give high efficiency. However, these high figures have nothing to do with the reality of the heat payer.

1. COMBUSTION EFFICIENCY

Combustion efficiency is the amount of fuel energy that is RELEASED during combustion.

The release of fuel energy and its conversion into heat in the hearth (stove) of the boiler room does not indicate the high efficiency of the boiler room. Combustion efficiency is provided by some boiler house manufacturers as boiler room efficiency, because 1) the figure is high (approximately 93-95%) 2) it is easy to measure combustion efficiency - you need to install the instrument in the chimneys.

The release of heat from fuel occurs in most boiler houses with high combustion efficiency.

Consequently: The release of fuel energy plus its conversion into heat in the hearth (stove) is not the same heat that is received by the boiler!! We are interested in the heat received by the boiler!!

2. BOILER ROOM efficiency

Boiler house efficiency is the amount of fuel energy that is usefully used, i.e. is transformed into another energy-carrying medium.

By other energy-carrying medium we mean, for example, warm water, which heats the house.

Boiler house efficiency is the most used definition of efficiency in all types of combustion plants.

Boiler room efficiency is more difficult to measure than combustion efficiency, so many people are content with only measuring combustion efficiency. In fact, the boiler room efficiency is 10-15% lower than the combustion efficiency.

3. EFFICIENCY OF COMBUSTION EQUIPMENT

THE EFFICIENCY OF COMBUSTION EQUIPMENT SHOWS HOW EFFECTIVELY COMBUSTION AND HEAT RECEPTION OCCUR IN THE BOILER ROOM. Even these calculations are often presented as a result of flue gas analysis.

Often, the efficiency of furnace equipment is used as an approximate analogue of the efficiency of a boiler room, since the measurement technique in this case is easier. Using this technique, you can obtain an approximate figure for the efficiency of a boiler room: it is necessary to constantly analyze the composition of oxygen or CO2 in the combustion gases. Losses are subtracted, since, for example, some heat is present in the ash/slag (this is especially true for slag-forming fuels). As for liquid fuel, the efficiency of furnace equipment and the efficiency of the boiler room are approximately the same, since liquid fuel does not contain ash/slag. But if you use this concept for coal or biofuels, then the errors (errors) are much higher.

4. EFFICIENCY OF THE INSTALLATION

When calculating the efficiency of an installation, the ratio between the total amount of useful energy and the total amount of energy is determined. IN total Energy also includes “auxiliary energy”, for example, electrical energy necessary for the operation of boiler room pumps, ventilation, chimneys, etc. For a liquid fuel installation, "auxiliary energy" corresponds to approximately 1% of the total fuel energy; for solid fuel installations, "auxiliary energy" equals 5% of the fuel energy.
The efficiency of the installation will thus be lower than the efficiency of the boiler room.

5. SYSTEM EFFICIENCY

Determining the efficiency of a system expands the boundaries of the system to:

Heat production with losses
- heat distribution with losses in heating mains, etc.
- heat use

According to UNICHAL (International Union of Heat Suppliers), the following typical losses in pipes when distributing hot water to apartments occur:

Sweden - 8% losses in pipes, i.e. heat is transferred to the ground and surrounding district heating pipes
Denmark - 20%
Finland - 9%
Belgium - 13%
Switzerland - 13%
West Germany - 11%

6. Annual efficiency

The efficiency per year in principle corresponds to the efficiency of the boiler house, but then the average efficiency of the boiler house is calculated for the entire year. The efficiency per year also includes periods with poor combustion levels, for example, when starting a boiler room, etc.

Efficiency per year depends on the size of the installation, service life, etc.

The above shows that they are used various definitions for efficiency, therefore there is a high probability that an erroneous figure will be given if the concept and definition of efficiency is not clarified. Thus, there is no need to be afraid of being insensitive, since in fact, many manufacturers, with or without knowledge, provide erroneous figures.

The important figures are those that reflect the real economic side of the fuel that the consumer buys. If you lose consumer trust due to providing too high efficiency, then the appearance big problems inevitable in the market.

As stated, "all suppliers" (at least many) give combustion efficiency when they offer boiler room efficiency information.

You cannot use combustion efficiency when calculating the economics of the installation!!!

THE CONSUMER IS NOT BUYING FUEL, BUT A MEANS FOR PRODUCING HEAT. It is not the fuel that should be cheap, but the heat that consumers receive during winter blizzards.

The value ranges from 0.3 to 3.5% and decreases with increasing boiler power (from 3.5% for boilers with a capacity of 2 t/h to 0.3% for boilers with a capacity of more than 300 t/h).

Loss of slags with physical heat occurs because when burned solid fuel The slag removed from the furnace has a high temperature: for solid slag removal = 600 °C, for liquid slag removal - = 1400 - 1600 °C.

Heat losses with physical heat of slag, %, are determined by the formula:

,

Where - the proportion of slag collection in the combustion chamber; - slag enthalpy, kJ/kg.

For layer combustion of fuels, as well as for chamber combustion with liquid slag removal = 1 – 2% and higher.

For chamber combustion of fuel with solid slag removal, the loss is taken into account only for multi-ash fuels at > 2.5%∙kg/MJ.

Boiler unit efficiency (gross and net).

The efficiency of a boiler unit is the ratio of the useful heat used to produce steam (hot water) to the available heat (heat entered into the boiler unit). Not all the useful heat generated by the boiler is sent to consumers; part of it is spent on its own needs (drive of pumps, draft devices, heat consumption for heating water outside the boiler, its deaeration, etc.). In this regard, a distinction is made between the efficiency of the unit based on the heat generated (gross efficiency) and the efficiency of the unit based on the heat supplied to the consumer (net efficiency).

Boiler efficiency (gross), %, can be determined by the equation direct balance

,

or equation reverse balance

.

Boiler efficiency (net), %, according to the reverse balance is determined as

where is the relative energy consumption for own needs, %.

Topic 6. Layer combustion devices for burning fuel in a dense and boiling (fluidized) bed

Furnaces for burning fuel in a dense layer: principle of operation, scope of application, advantages and disadvantages. Classification of furnaces for burning fuel in a dense bed (non-mechanized, semi-mechanical, mechanical). Fuel throwers. Mechanical fireboxes with moving grates: principle of operation, scope of application, varieties. Layer combustion devices for burning fuel in a fluidized bed: principle of operation, scope of application, advantages and disadvantages.

Layer combustion devices for burning fuel in a dense layer.

Layer furnaces, designed for burning solid lump fuel (size from 20 to 30 mm), are easy to operate and do not require a complex, expensive fuel preparation system.

But since the process of burning fuel in a dense layer is characterized by a low combustion rate, inertia (and, therefore, it is difficult to automate), reduced efficiency (fuel combustion occurs with large losses from mechanical and chemical underburning) and reliability, layer combustion is economically feasible for boilers with steam capacity up to 35 t/h.

Layered furnaces are used for burning anthracite, hard coal with moderate caking (long-flame, gas, lean), brown coal with low moisture and ash content, as well as lump peat.

Classification of layer fireboxes.

Maintenance of a furnace in which fuel is burned in a bed is reduced to the following basic operations: supplying fuel to the furnace; stirring (mixing) the fuel layer in order to improve the conditions for supplying the oxidizer; removal of slag from the furnace.

Depending on the degree of mechanization of these operations, layer combustion devices can be divided into non-mechanized (all three operations are performed manually); semi-mechanical (one or two operations are mechanized); mechanical (all three operations are mechanized).

Non-mechanized layered fireboxes are fireboxes with manual periodic supply of fuel to a fixed grate and manual periodic removal of slag.

Semi-mechanical combustion devices are distinguished by the mechanization of the process of supplying fuel to the grate using various throwers, as well as the use of special slag removers and rotary or swinging grates.